Vehicle exhaust systems direct exhaust gases generated by an internal combustion engine to the external environment. These systems are comprised of various components such as pipes, converters, catalysts, filters, etc. The overall system and/or the components are capable of generating undesirable noise as a result of resonating frequencies. Different approaches have been used to address this issue.
For example, components such as mufflers, resonators, valves, etc., have been incorporated into exhaust systems in an attempt to attenuate certain resonance frequencies generated by the exhaust system. The disadvantage of adding additional components is that it is expensive and increases weight. Further, adding components introduces new sources for noise generation.
Another approach utilizes a series of holes formed within a pipe that are covered with a microperforated material to dampen noise. In order to achieve the desired noise attenuation, the holes have to be relatively large in size. One disadvantage with this configuration is that the microperforated material is very thin and is not as structurally sound as the solid pipe wall. If large holes are cut into the pipe and covered with the microperforated material, the durability of the pipe may be adversely affected. Another concern is with grazing flow that occurs across the surface of the microperforated material. The acoustic properties of perforated material will change when exhaust gas flows across the surface of the material. This can often reduce the ability of the acoustic wave to propagate through the perforations, which limits the damping effect.